The present invention relates to a work-contacting probe system for a coordinate-measuring instrument, wherein a probe head supplies a work-contact signal and has a chuck for removable attachment of a selected one of a plurality of probe pins.
Probe heads of the switching type supply an output signal pulse at the instant when the shaped contact element of the probe pin of the probe head contacts the workpiece to be measured. A probe head of this type is illustratively described in U.S. Pat. No. 4,177,568. To increase the accuracy of workpiece-contact recognition, this known probe head has a piezoelectric sensor which is sensitive to very small contact force and thus responds to initial contact with the workpiece, by supplying a signal to retain the instantaneous measurement values of the instrument, namely, as instantaneously read from the measurement scales of the coordinate-measuring instrument. A second signal pulse is produced when the movable probe-pin holder moves out of its position of rest in the further course of the contacting movement. This second pulse serves to verify the first contact pulse and to discriminate from such irrelevant pulses as may be caused by machine vibrations.
Use of an adapter enables different probe-pin combinations to be selectively mounted to the movable chuck of this known probe-head, illustratively as described in U.S. Pat. No. 4,637,119, as well as in the present Assignee's Product Information Bulletin 60-20-027e, entitled "Probe Changing Systems" (Publication date III/85).
In spite of its versatility, this known probe system cannot satisfactorily solve all measurement tasks which are encountered in practice. For example, when a measurement task requires a very long or a very heavy probe-pin combination, a high starting or braking acceleration in the course of moving the probe head can momentarily dislodge the probe-pin adapter from its seated (chucked) position, thus erroneously simulating a contacting process.
To handle measurement tasks which require large probe-pin lengths, it is known to provide, in place of a central switching-type probe head which can carry a selected one of various probe-pin combinations, a plurality of small switching probe heads, each of which has only a very short probe pin; in turn, each of the small switching probe heads is mounted to the coordinate-measuring instrument, via suitable distributor or extension means. Such a probe system is known, for example, from West German Patent Application P 36 34 689.6 or from the corresponding pending U.S. Pat. Application Ser. No. 105,825 as well as from applicant's Assignee's "Information Brochure" (60-25-004e), in connection with the "Microtecnic 86" Exposition in Zurich, Switzerland, October 14 to 18, 1986. As can be noted from FIGS. 4 to 6 on Pages 12 and 13 of the said brochure, these small switching probe heads of smaller diameter are mounted on the coordinate-measuring instrument via a buckling (yieldable) point, for protection against collision damage. Collision protection is afforded by a collision-sensing switch which is connected to the control system of the coordinate-measuring instrument.
Such collision protection is, however, biased with relatively great spring force, as compared with bias of the probe-pin holder in a probe head of the first-mentioned type (e.g., as in U.S. Pat. No. 4,177,568). This is necessary in order to assure retention of the zero position of the currently mounted probe-head combination, with its relatively great mass, even if a relatively great rotational moment should act on the collision-protection device, as a result of an eccentric force application.
When a given system employs a large number of small switching probe heads, it becomes relatively expensive to provide collision protection for all heads, and the price of a thus-equipped coordinate-measuring instrument necessarily increases. Furthermore, a simple change between a single probe-head system and a multiple probe-head system is not possible since an entire multiple-head system, including all provision for its collision protection, must be removed if a single switching probe head is to be mounted in its place. Such conversions always mean subsequent recalibration of the involved probe system.